The most commonly used non-medical treatment for Irritable Bowel Syndrome (IBS) is a combination of traditional stress management techniques and non-specific cognitive therapy methods. This general approach is used to treat patients with divergent target symptoms (i.e., constipation and diarrhea). A limitation of this approach is that the treatment is not derived from a clear theoretical model that identifies a specific process underlying the disorder or treatment. The present study is based on a strong theoretical model that offers specific predictions and treatment methods. The model assumes that hypervigilance and hypersensitivity to internal sensations in the gut are the critical perceptual mechanisms underlying IBS. These perceptual filters are the result of learning and conditioning processes and can be reversed with a cognitive-behavioral treatment that incorporates gradual exposure to internal sensations. Subject recruitment will be limited to IBS patients with diarrhea as the predominant symptom and the treatment will be implemented with a multiple baseline across subject design. The treatment will consist of several phases: education about the role of learning in the development of IBS symptoms; relaxation training; cognitive restructuring for overestimation and catastrophization errors; exposure to the feared internal sensations; and exposure to avoided situations. The goal of this treatment is to reverse the hypersensitivity and hyperviligance through gradual exposure to these stimuli. The broad objective of this research is to enhance the treatment for IBS and to test the predictions of this theoretical model.